The invention disclosed and claimed herein generally pertains to an RF coil arrangement for use with a magnetic resonance (MR) imaging system in the direction of the B.sub.0 field. More particularly, the invention pertains to an RF coil arrangement of the above type which generates I and Q quadrature components to produce a circular polarized RF field within the imaging space. Even more particularly, the invention pertains to an RF coil arrangement which is particularly useful in connection with an MR system of the above type for applications such as spine imaging.
As is well known, in a bore-type full body MR imaging system, a patient is placed within the bore of a cylindrical main magnet. The static or main B.sub.0 magnetic field is oriented along the bore axis, and therefore is oriented in a direction extending between the patient's head and feet (hereinafter Superior-Inferior or SI direction). However, in recent years innovators in the MR field have developed and introduced interventional or open magnet MR imaging systems. In such systems the DC or main B.sub.0 field is produced by two spaced-apart magnet components. The imaging space, that is, the space in which a patient or other object resides during the imaging procedure, is located between the main magnet components.
In a number of such systems, known as side-entry systems, a patient enters the imaging space by passing between the main magnet components, and then proceeds to sit, lie or stand therebetween. Thus, the main B.sub.0 field has a lateral orientation, that is, is oriented from side to side (hereinafter Left-Right or LR direction) with respect to a patient. Moreover, in such arrangements it may be very useful to position the two main magnet components fairly close together. This may be done where the spacing between the two components only needs to accommodate the dimension of a patient taken along the LR or AP direction, and not along a patient's SI direction.
While it is anticipated that open magnets of the above type will provide significant advantages over cylindrical full-body magnets, certain challenges are presented thereby, for applications such as large field-of-view (FOV) spine imaging. In spine imaging, it has been common practice to provide two RF quadrature components, that is, components having a 90.degree. phase difference between them, to generate a circular polarized RF field. This has been done to achieve acceptable SNR or sensitivity in spine imaging. In the past, a butterfly-loop combination coil has typically been employed to generate the RF components. Such combination may comprise separate single loop and butterfly coils, or alternatively may comprise a single resonating coil structure. The single loop produces an RF component orthogonal to the plane of the coil (vertical mode) and the butterfly coil produces an RF component which is parallel to the coil plane (horizontal mode). As is well known, the plane of the circular polarized field generated by the two RF components must be orthogonal to the direction of the B.sub.0 field.
A disadvantage of butterfly-loop coils of the prior art is that such coils tend to generate RF fields having bright spots at different locations in the circular polarized plane. This is especially bothersome when B.sub.0 is LR and bright spots favor certain vertebrae over others. Bright spots are locations at which RF field intensity is significantly greater than at adjacent surrounding locations. Accordingly, bright spots introduce nonuniformities or inhomogenities into the RF field, between different positions lying in a plane orthogonal to the main B.sub.0 field. In previous spine imaging procedures, carried out in association with a bore-type MR imaging arrangement or the like, the RF bright spots generally were not a serious problem. This is because a patient's spine in such imaging arrangement would be directed along the B.sub.0 field and would not be affected by RF inhomogenities between different points along a line perpendicular to the B.sub.0 field. In fact, it could be advantageous to align the spine with an RF field bright spot, which likewise extended along the B.sub.0 field. Now, however, in the open magnet systems described above, spine imaging must be carried out wherein the B.sub.0 field is oriented LR with respect to the patient. Thus, the spine lies in the plane of the circular polarized RF field, and acquired spine images could be significantly affected by the bright spots, if a prior art butterfly-loop coil were to be employed to generate the RF field. For example, one or two vertebrae of the spine could coincide with an RF bright spot, while the remaining vertebrae were at very different RF intensities. Accordingly, it is necessary to provide an alternative RF coil structure which can generate a circular polarized RF field, from which the bright spot inhomogenities of the prior art are eliminated.